A comprehensive analysis has been made of the mortality data published in tabular form in great detail by the Society of Actuaries in the Build Study 1979. Data are available on 5 age decades (20-29 through 60-69 yr) on both men and women. Mortality ratios (actual/expected deaths) were related to a large number of height-weight categories. These data provide the source for the computation of the widely accepted Metropolitan 1983 Tables. Examination of the Build Study data showed that the relation of the Body Mass Indices (BMI = Wt/Ht2) to the Mortality Ratios (MR) was U-shaped and that a quadratic equation was required to fit the data accurately. Computations for each age-sex group were based on the following equation: MR = a + b (BMI) + c (BMI)2. The equation (1) permits the construction of curves which accurately relate BMI to MR; (2) provides the value of BMI associated with minimal mortality (the nadir of the U-shaped curve); and (3) provides the range of BMI's associated with less than expected mortality (the two points at which the U-shaped curve intersects the 100 MR line). The BMI of minimal mortality was found to increase markedly with age. Furthermore, there were no significant differences between men and women. Thus the "best" BMI increases from 20.5 kg/m2 at age 20-29 yr to 26.9 at age 60-69yr. The recommended range of BMIs can be set at 18-23 kg/m2 for 20-29 yr and increases linearly to 24-30 kg/m2 for 60-69 yr old subjects. These vallues contrast sharply with those previously recommended (20-25). The 1983 Metropolitan Tables, (when heights and weights are converted to BMI) recommend essentially a range of 20-26 for men and women from age 25-59 years. The data show however that age cannot be neglected as a variable in the setting of recommended weight ranges.